1. Field of Invention
The present invention relates to a cancellation circuit that suppresses electromagnetic interference in a high speed circuit and, in particular, to a cancellation circuit that cancels the magnetic field.
2. Related Art
To increase the electromagnetic compatibility (EMC) of electronic products, one has to take account of the circuits of the power, signal lines, analogue signals, digital signals, electronic box and operational box when designing the products. One crucial thing is the filter circuit, including the decoupling capacitor, the filter capacitor, the bypass capacitor, etc. Since the clock rate on the PC board is increasing (from 33 MHz to 133 MHz), normal filters imposed on signals (including RC filters and LC filters) cannot achieve the object of canceling electromagnetic interference in a high speed circuit under the consideration of the possibility of affecting the signal quality.
In observation of the fact that the above-mentioned filters will serious affect the signal quality of a high speed circuit, it is a feasible way to use differential signals to cancel electromagnetic interference in a high speed circuit. However, most current high speed circuit signals are single-ended signals, so their electromagnetic interference cannot cancel with that of the differential signals.
The cancellation circuit disclosed herein to suppress the electromagnetic interference of a high speed circuit invokes another circuit by the high speed circuit to generate differential signals for canceling the magnetic field and coupling to the electric field. The differential signal is generated by a dummy signal that passes through a phase shifter, thus having the differential effects.
The additional circuit provided by the invention includes a same-layer type, a meander type and a different-layer stacking type.